1. Field of the Invention
The invention relates to a method of displaying an image on a display device in a plurality of periods called sub-fields, where the display device is capable of generating, in each of the sub-fields, a respective illumination level, the method comprising the steps of:
generating a set of combinations of sub-fields, each element of the set representing a respective available illumination level,
selecting, for pixels of the image, particular combinations of sub-fields from the set in conformity with the intensity value of the respective pixels, and
sending, for each of these pixels, a representation of the selected combination of sub-fields to the display device in order to display the particular pixel.
The invention also relates to an image processing unit for processing an image to be displayed on a display device in a plurality of periods called sub-fields, wherein the display device is capable of generating in each of the sub-fields a respective illumination level, the image display unit comprising:
storage means for storing a set of combinations of sub-fields, each element of the set corresponding to a respective available illumination level,
selection means for selecting from the set a particular combination of sub-fields in conformity with the intensity value of a particular pixel of the image, and
sending means for sending a representation of the selected combination of sub-fields to the display device in order to display the particular pixel.
The invention also relates to an image display apparatus comprising such an image processing unit.
2. Description of the Related Art
The European Patent Application Number EP 884 717 A1, corresponding to U.S. Pat. No. 5,841,413, describes a plasma display panel driven in a plurality of sub-fields. A plasma display panel is made up of a number of cells that can be switched on and switched off. A cell corresponds to a pixel (picture element) of the image that is to be displayed on the panel. Three phases can be distinguished in the operation of the plasma display panel. The first phase is the erasure phase in which the memories of all cells of the panel are erased. The second phase is the addressing phase in which the cells of the panel that are to be switched on are conditioned by setting appropriate voltages on their electrodes. The third phase is the sustain phase in which sustain pulses are applied to the cells which cause the addressed cells to emit light for the duration of the sustain phase. The plasma display panel emits light during this sustain phase. The three phases together are called a sub-field period, or simply a sub-field. A single image, or frame, is displayed on the panel in a number of successive sub-field periods. A cell may be switched on for one or more of the sub-field periods. The light emitted by a cell in the sub-field periods in which it was switched on is integrated in the eye of the viewer who perceives a corresponding intensity for that cell. In a particular sub-field period, the sustain phase is maintained for a particular time, resulting in a particular illumination level of the activated cells. Typically, different sub-fields have different durations of their sustain phase. A sub-field is given a coefficient of weight to express its contribution to the light emitted by the panel during the whole frame period. An example is a plasma display panel with 6 sub-fields having coefficients of weight of 1, 2, 4, 8, 16 and 32, respectively. Selecting the appropriate sub-fields in which a cell is switched on, enables 64 different intensity levels to be realized in displaying an image on this panel. The plasma display panel is then driven by using binary code words of 6 bits each, such a code word indicating the intensity level of a pixel in binary form.
In driving a plasma display panel, the frame period, i.e., the period between two successive images, is separated into a number of sub-field periods. During each of these sub-field periods, a cell may or may not be switched on and integration over the sub-field periods results in a perceived intensity level of the pixel corresponding to this cell. Instead of displaying a pixel at a given moment in time, on a plasma display panel, the pixel seems to be displayed as a series of sub-pixels shifted in time with respect to each other. This may cause artifacts if a series of images contains a moving object. The eyes of the viewer track the moving object, while the elements of the object emit light at various different moments. These temporal differences between parts of the object are translated into spatial differences by the tracking eye, resulting in artifacts, like false contours. Another artifact is motion blur. Motion blur occurs if the intensity level of the pixels of a moving object is generated in a large number of sub-fields. It is then clearly noticeable that the light of a pixel has been emitted at the various different moments.
The motion of an object needs to be taken into account when displaying the object in a number of sub-fields. For each next sub-field, the object must be moved a little. Motion compensation techniques are used to calculate a corrected position for the sub-pixels in the sub-fields. In some circumstances, the motion compensation is not fully reliable and may produce erroneous results, for example, in an area of the image containing little detail. The erroneous results lead to motion compensation where this should not be done. This also gives motion artifacts that are very visible.
An artifact is most noticeable if two neighboring pixels have a small difference in intensity level while, for one of the pixels, the sub-field with the largest coefficient of weight is switched on and, for the other pixel, this sub-field is switched off. In case of the example of the binary code above, the code word for one pixel has the most significant bit on and the code word for the other pixel has the most significant bit off. Any error in the calculated position of a sub-field, i.e., any motion artifact involving these pixels, will then cause a relatively large artifact in the displayed image. An example of the occurrence of a motion artifact in the plasma display panel with 6 sub-fields is the transition from intensity level xe2x80x9831xe2x80x99 to intensity level xe2x80x9832xe2x80x99. The level xe2x80x9831xe2x80x99 has the 5 lower sub-fields switched on and the highest sub-field switched off. For the level xe2x80x9832xe2x80x99, the 5 lower sub-fields are switched off and the highest sub-field is switched on. This causes a very visible artifact if there is motion involved. The device described in EP 884 717 A1 tries to mitigate motion artifacts by restricting the code words that are used. This known device employs more sub-fields than necessary for realizing the required set of intensity values. The resultant set of code words for expressing the intensity value is redundant, i.e., for a given intensity value, more than one code word is available. From this redundant set, there is formed a subset for which those code words are selected that give the fewest motion artifacts for expressing a difference between the intensity values. This subset is created by searching the original set and determining what the effect on the artifacts may be for a difference between a given code word and each of the other code words.
It is an object of the invention to provide a method, as described in the preamble, which offers an improved reduction of artifacts. This object is achieved according to the invention by a method that is characterized in that the set is generated while limiting a difference regarding sub-fields between a first one of the combinations representing a first available illumination level and a second one of the combinations representing the next higher illumination level in the set, the limiting including control such that only a limited number of the sub-fields that are switched on in the first one of the combinations are not switched on in the second one of the combinations. For creating a set with a comparatively large number of different available illumination levels, it is desirable that the combination for the next higher level may have a number of sub-fields switched off that are switched on for the given level. This provides an amount of freedom to create the comparatively large number of different levels. Limiting the number of sub-fields that are switched off for the next higher level ensures that the set of combinations of sub-fields, according to the invention, will suffer less from dynamic false contours. As described above, an area with a small spatial graduation, i.e., an area where neighboring pixels have a very small mutual difference in intensity level, may suffer heavily from motion artifacts, like false contours. Because, in such an area, the invention effectively controls the mutual differences in sub-fields between neighboring pixels, the chance of motion artifacts is reduced. There are fewer sub-fields that change value between pixels and, therefore, fewer timing errors leading to the artifacts are likely to occur.
An embodiment of the method according to the invention, wherein the limiting includes control such that only two of the sub-fields that are switched on in the first one of the combinations are not switched on in the second one of the combinations, provides a good balance between the number of available combinations of sub-fields and the reduction of dynamic false contours in the case of motion.
A further embodiment of the method according to the invention, wherein the limiting includes control such that only one of the sub-fields that are switched on in the first one of the combinations is not switched on in the second one of the combinations, provides a good balance between the number of available combinations of sub-fields and the reduction of dynamic false contours in the case of motion at comparatively high speeds.
Another embodiment of the method according to the invention, wherein a first sub-field is switched on in the first one of the combinations and not switched on in the second one of the combinations, wherein a second sub-field is not switched on in the first one of the combinations and switched on in the second one of the combinations, and wherein the first sub-field and the second sub-field are temporally adjacent, motion artifacts as described above are reduced further. Any difference in time between a pixel of a given intensity level and a pixel of the next higher level will be small, thus further reducing the chance of a motion artifact.
A further embodiment yet of the method according to the invention notes that it is advantageous to generate the available intensity levels in such a way that they are uniform in the perception of the viewer. The reduced number of levels, when compared with a binary distribution, is thus used efficiently in view of the perceived quality of the image.
In another embodiment of the method according to the invention, the perceptual scale is substantially defined according to the function L=xY, in which L is the perceived luminance, x is the number of the available illumination level in the set, and Y is a constant of a value between 2 and 3. This distribution of available intensity levels corresponds to the inverse of the gamma filtering that is applied to video signals produced by a camera. Therefore, this embodiment does not require the separate step of inverse gamma filtering as applied in the known method.
In a further embodiment of the method according to the invention, a complementary set of combinations of sub-fields is generated to increase the number of available illumination levels, which complementary set is not limited regarding the changes between particular ones of the combinations, the original set and the complementary set together forming an overall set of available illumination levels, wherein it is examined whether there is motion between the image and a preceding image, and wherein, if motion is found to be present, the particular combination of sub-fields is selected from the original set, and if no motion is found to be present, the particular combination of sub-fields is selected from the overall set. This version allows that the combination of sub-fields for a pixel of a still image is selected from an overall set containing a large number of available illumination levels and that the combination of sub-fields for a pixel from an image containing a moving object is selected from a set with a limited number of available illumination levels suffering less from motion artifacts. In this way, a still image which will not suffer from motion artifacts since there is no motion, is displayed with a large number of intensity levels whereas only an image with motion is displayed with the reduced number intensity levels.
In yet another embodiment of the method according to the invention, a complementary set of combinations of sub-fields is generated to increase the number of available illumination levels, which complementary set is not limited regarding the changes between particular ones of the combinations, the original set and the complementary set together forming an overall set of available illumination levels, wherein it is determined whether a particular object or area in the image is in motion between the image and a preceding image, and wherein for pixels of the moving object the particular combination of sub-fields is selected from the original set and for pixels of the image that do not belong to the moving object the particular combination of sub-fields is selected from the overall set. According to this version only the moving object itself is displayed with the reduced number of intensity levels while the non-moving parts of the image are displayed with the higher number of intensity levels.
It is a further object of the invention to provide an image processing unit as described in the preamble which offers an improved reduction of artifacts. This object is achieved according to the invention by an image processing unit that is characterized in that, in the set, a difference regarding sub-fields between a first one of the combinations representing a first available illumination level, and a second one of the combinations representing the next higher illumination level in the set, has been limited, the limiting including control such that only a limited number of the sub-fields that are switched on in the first one of the combinations are not switched on in the second one of the combinations.